Forwarders' hopes, was SPF and bouncing

On Sun 08/Apr/2012 11:36:31 +0200 alan wrote:
> At 15:05 07/04/2012  Saturday, Michael Deutschmann wrote:
>>
>> We aren't discussing forwarders, but ordinary SMTP with only one
>> hop across the border.

I like how SPF works in those cases.

>> The only sustainable hope for a forwarder is a side deal with the
>> admins of the ultimate recipient server to obtain the same
>> super-whitelisting (overriding even mailbox diskspace quota and
>> immune to automated blacklisting) that backup MXes have.  Then it
>> really doesn't matter what the MAIL FROM: is.

What about "generic" forwarders?  I mean sites who provide a web-form
to let users specify whatever address they like.  They are obviously
not going to hound ultimate servers' admins in order to have their
site whitelisted.  BTW, most target domains are gmail, yahoo, and the
like, so such whitelisting is probably tantamount a whitelist of all
legitimate mail servers, which is probably what those huge mailbox
providers are doing anyway.

Small to normally large mail sites cannot be reliable forwarding
targets if they reject when an smtp.mailfrom results in a fail.
So, I think it would be fair to require to also check smtp.helo in
such cases, and accept the message if the it results in a pass.  What
do you think?

> ok this is the one time bounces MAY legitimately occur (disk
> quotas) but unless the user side is abandoned or badly run it
> shouldn't be persistent or even a common enough occurrence to cause
> trouble (also sys-admins are supposed to be aware of these issues
> to lart/delete the user before these issues happen) (as far as I am
> aware no backscatter RBLs consider mailbox full as a listing
> offence even if to innocent 3rd party due to this) a well setup
> forwarder or frontline MX will cache 5xx and 2xx responses for
> period (thus a 5xx from overquota) will result in 1 bounce every
> period ONLY) obviously it also extends the period the forwarder or
> frontline MX rejects incoming mail beyond the time the user has
> resolved the issue, in our case 15mins as we see most users pickup
> their mail at this interval.

After a given number of bounces, the forwarding could well be removed.
That's the approach mailing lists do, and provides for automatically
removing stale addresses.  Are there implementations doing so?